1. Field of the Invention
This invention relates to processes for producing patterned high T.sub.c superconducting layers, and more particularly to techniques that enable the writing of any type of pattern of high T.sub.c superconducting materials where the pattern is produced prior to the formation of a high T.sub.c superconducting phase in the materials.
2. Description of the Related Art
High T.sub.c oxide superconductors are materials of the type first discovered by J. G. Bendnorz and K. A. Mueller and reported by them in Z. Phys. B, 64, 189 (1986). These are superconducting oxides typically including combinations of 1 or more rare earth elements, alkaline earth elements, copper and oxygen and in which the transition temperature is greater than 30K. Typical high T.sub.c superconducting oxides are those fabricated from compounds of La, Sr, Cu and O, or Y, Ba, Cu and O. One of these materials, the Y-Ba-C-O oxide superconductor, has exhibited critical transition temperatures in excess of 77K. A particularly preferred single phase composition of this material is Y.sub.1 Ba.sub.2 Cu.sub.3 O.sub.y, which is often referred to as a "1-2-3" superconducting phase.
In the electronics industry, the fabrication of films of various thicknesses is important. In particular, the deposition of superconducting YBa.sub.2 Cu.sub.3 O.sub.7-y thin films has been obtained by various techniques, including sputtering, evaporation, and plasma spray coating. Related copending applications describing vapor transport and plasma spray coating of high T.sub.c superconducting oxides are Ser. No. 027,584, filed Mar. 18, 1987 and Ser. No. 043,523, filed Apr. 28, 1987, respectively. In addition, reference is made to the following technical journal articles which also describe the deposition of superconducting films.
1. R. B. Laibowitz et al, Phys. Rev. B, 35, 8821 (1987). PA1 2. P. Chaudhari et al, Phys. Rev. Lett., 58, 2684 (1987). PA1 3. J. J. Cuomo et al, submitted to the American Ceramics Bulletin.
In thin film technology, it is necessary to provide patterns of the films when devices, interconnections, and packages are to be fabricated. In the case of high T.sub.c oxide superconductors, it has become clear in the art that patterning of these materials is not trivial. Generally, the materials are ceramic copper materials having a perovskite-like structure that is not easily patterned. Wet photolithographic methods involve the use of various chemicals to which these ceramic materials are very sensitive, thus leading to alteration of their superconducting properties. Additionally, these materials tend to be porous and the use of chemicals will lead to etching of regions under an applied resist mask, thereby leading to poor resolution and undercutting.
Negative patterning of thin high T.sub.c superconducting films can be done by ion implantation as described by G. C. Clark et al, Appl. Phys. Lett. 51, 139 (1987). This technique utilizes ions to destroy the superconductivity in the irradiated regions when the ion implantation is above a threshold dose. A superconducting quantum interference device (SQUID) was fabricated in this way and has been described by R. H. Koch et al in Appl. Phys. Lett. 51, 200 (1987). Additionally, this device and its fabrication technique are described in copending application Ser. No. 037,912, filed April 13, 1987, and assigned to the present assignee. The ion implantation technique is, however, limited to very thin layers of up to about 1-2 micrometers and also involves a high vacuum that tends to deplete oxygen from the superconducting film. Therefore, an annealing step is required after ion implantation. This may lead to diffusion of the implanted ions which could affect superconductivity in the film.
Another approach to patterning high T.sub.c superconducting films is laser ablation using an appropriate mask either in contact with the film, or by projection imaging. This type of technique has been described by M. Scheuermann et al in an article submitted to Appl. Phys. Lett. In this technique, patterning is obtained by removing the unwanted areas of the film by direct ablation using an excimer laser providing ultraviolet wavelengths. This technique has limitations in that mask fabrication is required and the process itself produces debris which must in some way be removed.
In order to improve the patterning of layers of high T.sub.c superconductors, a discovery has been made which allows patterning to occur in a fabrication step prior to the achievement of a superconducting thin film. This inventive technique does not require the use of a mask and allows direct writing with an energy beam to accomplish patterning of any arbitrary geometry.
Accordingly, it is an object of this invention to provide an improved technique for producing patterns of high T.sub.c superconducting layers.
It is another object of this invention to provide a technique for patterning high T.sub.c superconducting layers which does not require that the layers be patterned after they are in a superconducting state.
It is another object of the present invention to provide a process that produces patterned high T.sub.c superconducting layers, where direct writing using an energy beam can be used to provide any desired pattern.
It is another object of the present invention to provide a technique for producing patterns of high T.sub.c superconducting layers, where the technique is not limited by the physical and chemical properties of the superconducting material.